i#3544 RISC-V, part 6: Add extension stubs

Created by: semihalf-kardach-stanislaw

Add definitions required for extensions to compile for RISC-V.

NOTE: This code is not validated and mostly contains stubs as the main point was to achieve compilation and estimate the effort required for the port. Some of the trivial logic has been implemented though.

Issue: #3544

Signed-off-by: Stanislaw Kardach kda@semihalf.com